The term “force-measuring cell” means a measurement transducer which converts a force presented as an input quantity for example into an electrical signal as an output quantity. Specifically, a force-measuring cell designed as a weighing cell serves to perform the electro-mechanical conversion of the weight force exerted by a weighing object into an electrical signal.
Many force-measuring devices, in particular gravimetric measuring instruments such as for example balances, thermo-gravimetric instruments, measuring instruments for gravimetric moisture determination, weighing modules for tank installations and reactor vessels, weighing modules and multi-module systems in filling- and packaging machines, and also torque-acceleration-measuring devices are in some cases used in very aggressive environments, and effective measures are therefore taken to protect them against destruction. Such protective measures include housings adapted to the operating environment which have to meet requirements in regard to the penetration of dust, moisture and the like, as codified for example under the so-called ingress protection ratings according to the European Norm Standard EN 60529.
The housing has an interior space in which sensitive components are arranged, such as electro-mechanical measurement transducers, sensors and electronic components for the signal processing circuits, and also sensitive mechanical devices such as for example force-transmitting devices or lever mechanisms. In a force-measuring device which generates a counter-acting force in opposition to the external force, as for example in a force-measuring device that is based on the principle of electro-magnetic force compensation, the force-generating device and its feedback control device are normally also arranged in the interior space of the housing, as these components are very sensitive to extraneous disturbances.
Furthermore, in the case of a force-measuring device of the last-mentioned type, the external force acting on the electromechanical measurement transducer that is arranged in the interior space as well as a counter-acting force are transmitted through a passage in the housing wall which may in some cases be configured as a diaphragm. This function is normally performed by a force-transmitting device such as a rod which should, however, be encumbered as little as possible by the housing.
High-capacity force-measuring cells for weighing modules, so-called tank- or reactor vessel weighing modules are installed for example in stainless steel housings that are closed off by gas-tight welds. The force-measuring cells enclosed in such housings operate without problems as long as the housing keeps the force-measuring cell isolated from ambient factors which have a harmful influence on the weighing signal. If there is a leak in the housing, the force-measuring cells are in most cases not instantly destroyed; rather, the destruction occurs gradually and is often noticed only at an advanced stage. If the measurement device is installed in industrial systems with a high degree of automation, a failure of the measuring device can lead to a long downtime period of the system or can cause defective products.
High-capacity force-measuring cells also find application as weighing modules in scales for trucks. Typically, a weighing system of this kind has several weighing platforms, each of which is supported by four weighing modules. These scales are normally used outdoors and are therefore particularly exposed to changing environmental conditions. In addition, the exposure entails the immediate risk that these weighing modules can be hampered in their ability to function correctly due to improper manipulations such as intentional, negligent or erroneous acts of mistreatment or intrusion.
Depending on the ambient conditions, the force-measuring devices do not necessarily have to be hermetically encapsulated. Less complicated and expensive housings with contact-free passages in the form of a labyrinth baffle as disclosed for example in DE 101 49 606 C2 can also be used in an industrial environment. Normal scale housings, too, will meet the purpose if they are compatible with the ambient conditions. However, in case of a manipulation error it is possible that for example a liquid may penetrate into the interior of the housing, whereby the relative humidity in the interior space of the housing may increase to such an extent as to cause corrosion of the force-measuring cell or electronic components of the signal-processing circuitry.
In many instances, force-measuring devices are transported over large distances and intermediately stored before being put into service. If the devices are exposed to inappropriate ambient conditions during transportation and storage, it is possible that condensation will occur in the interior of the housing which can critically compromise the measurement performance.
To conduct an inspection of the force-measuring cell involves a major undertaking or may even be impossible, depending on the housing in which the force-measuring cell is enclosed. A periodic examination of the force-measuring devices used in systems is complicated and expensive to perform.
A force-measuring cell which is disclosed in EP 1 347 277 B1 is additionally equipped with a temperature sensor. The analog signals of the force-measuring cell are converted by means of a first converter circuit and the analog signals produced by the temperature sensor are converted by means of a second converter circuit into two-level, pulse width-modulated signals. These signals are transmitted through connector leads to a processor module where they are further processed by means of compensation data that can be called up from a memory module. As a result of processing the force-measuring cell signals with the temperature sensor signals, the temperature drift of the weighing cell is corrected. While this kind of treatment of the weighing signal provides an adequate compensation for the effects of the ambient environment on the weighing signal, the actual condition of the force-measuring cell cannot be determined by it.